Conventionally, for high-energy density electric storage devices as typified by lithium ion secondary batteries, sheet-like current-collecting foil (such as aluminum foil or copper foil) rolled up in the form of a roll is passed through a die coater, a comma coater, or the like to coat the current collecting foil with an active material (such as a lithium composite oxide or carbon), thereby preparing sheet-like electrodes.
Further, in order to prevent short circuits between the sheet-like electrodes in contact with each other, separators are interposed between the electrodes, the electrodes and the separators are wound or stacked as sheets to prepare electrode groups, and aluminum tubs or nickel tubs are welded as external terminal electrodes to the electrodes by a method such as ultrasonic welding so as to be electrically connected to the electrodes.
Then, the thus prepared elements composed of the electrodes, the separators, etc. are housed in an aluminum can, a sac-like exterior material composed of an aluminum laminate film, or the like, and subjected to sealing after injecting an electrolyte solution, thereby preparing electric storage devices.
Now, remarkable progress has been made on such electric storage devices, but in recent years, increased reliability of electric storage devices for cycle characteristics, and charge-discharge rate characteristics improved by lower resistance have been further required as typified by storage batteries for hybrid automobiles.
Batteries that use lithium titanium oxides for negative electrode active materials have been examined as such electric storage devices. The lithium titanium oxides for negative electrode active materials are hardly deteriorated by expansion or shrinkage of the crystal structure due to a small change in crystal lattice volume with charge and discharge, further, the reaction between the negative electrode and the electrolyte solution is inhibited because the potential for storing and releasing lithium ions is high and +1.55 on the basis of Li/Li+, and reliability for cycle characteristics and the like is known to be improved as compared with cases of using carbon such as graphite for the negative electrode active materials.
Further, as a technique for further improving high-temperature reliability, Patent Document 1 suggests a non-aqueous electrolyte lithium secondary battery basically composed of a negative electrode mainly including a spinel-type lithium titanium oxide, a positive electrode that has a higher potential than that of the spinel-type lithium titanium oxide, and an organic electrolyte solution, where the electric capacity of the negative electrode is made lower than the electric capacity of a chargeable/dischargeable region of the positive electrode. More specifically, Patent Document 1 presents a non-aqueous electrolyte lithium secondary battery which has high-temperature reliability improved by making the negative electrode capacity (mAh) lower than the positive electrode capacity (mAh).
In addition, Patent Document 2 suggests a non-aqueous electrolyte battery which has high-temperature reliability (cycle characteristics) improved by making the negative electrode capacity (mAh) higher than the positive electrode capacity (mAh), in contrast to Patent Document 1 mentioned above.
However, conventionally, batteries that use a lithium titanium oxide of spinel-type crystal structure for a negative electrode active material and a lithium transition metal oxide having a layered crystal structure for a positive electrode active material like the non-aqueous electrolyte lithium secondary battery in Patent Document 1 are known to have the problem of having cycle characteristics degraded particularly at high temperature.
On the other hand, Patent Document 2 mentions that when a positive electrode capacity ratio is made higher than a negative electrode capacity ratio, the balance in actual electric capacity between the positive electrode and the negative electrode is lost under environment at high temperature when the actual electric capacity of the negative electrode is less than the actual electric capacity of the positive electrode, because the negative electrode undergoes a larger increase in actual electric capacity with increase in temperature as compared with the positive electrode. This brings the positive electrode into a overcharge condition in spite of normal charge-discharge cycle, and results in dramatically degraded cycle characteristics (Patent Document 2, paragraph 0020).
Further, Patent Document 2 discloses high-temperature cycle characteristics improved by making the positive electrode capacity lower than the negative electrode capacity. For example, Patent Document 2 discloses a capacity maintenance ratio of 88% as a most favorable example as a result of carrying out a charge-discharge cycle test up to 300 cycles when 5C charge/1C discharge is repeated under an environment at 60° C. (Patent Document 2, Table 1).
In this regard, the lithium titanium oxide has a low packing density (3.5 g/cc), whereas the positive electrode active material having a layered crystal structure such as LiCoO2 or LiCo1/3Ni1/3Mn1/3O2 has a high packing density (4.6 to 5.0 g/cc).
In addition, the capacity of the positive electrode material and the capacity of the lithium titanium oxide both represent close values (for example, the lithium transition metal oxide having a layered crystal structure such as LiCoO2 or LiCo1/3Ni1/3Mn1/3O2 has a capacity of 150 to 170 mAh/g in the case of charging at 4.3 V and discharging at 2.7 V (vs. Li/Li+), and the lithium titanium oxide has a capacity of 166 mAh/g in the case of charging at 2.0 V and discharging at 1.0 V (vs. Li/Li+)).
Therefore, when a battery is prepared as described in Patent Document 2, there is a need to reduce the thickness of the positive electrode layer and increase the thickness of the negative electrode layer in order to increase the capacity of the negative electrode.
Further, in the case of such a composition, the spinel-type lithium titanium composite oxide is approximately two orders of magnitude lower in ion conductivity as compared with carbon materials for use in negative electrode active materials and lithium cobalt composite oxides (for example, LiCoO2) for use in positive electrode active materials, and there is thus a bias generated between a local load associated with a charge (discharge) reaction at the positive electrode and a local load associated with a charge (discharge) reaction at the negative electrode. Therefore, it is difficult to increase the reliability of the electric storage device under an environment at a high temperature such as 85° C., and improve charge-discharge rate characteristics by lowering the resistance.
In addition, even when the positive electrode capacity is made higher than the negative electrode capacity as described in Patent Document 1, the distance between the negative electrode layer and a current collector layer is longer when the thickness of the negative electrode layer is made, for example, 40 μm or more, the spinel-type lithium titanium composite oxide is approximately two orders of magnitude lower in ion conductivity as described above, there is thus a bias generated between a local load associated with a charge (discharge) reaction at the positive electrode and a local load associated with a charge (discharge) reaction at the negative electrode, and it is difficult to increase the reliability of the electric storage device under an environment at a high temperature such as 85° C., and improve charge-discharge rate characteristics by lowering the resistance.
Patent Document 1: Japanese Patent Application Laid-Open No. 10-69922
Patent Document 2: Japanese Patent Application Laid-Open No. 2007-273154